Fluid servo systems are used for many purposes, one being to position the flight control surfaces of high performance aircraft. In such an application, the servo system desirably should provide for control and damping of flight control surface displacements of flutter after loss of fluid power. Otherwise, aircraft damage or loss of control may result.
In conventional electro-hydraulic systems, electro-hydraulic valves have been used in conjunction with servo valve actuators to effect position control of the main control servo valve. Typically, the servo actuators in redundant systems operate on opposite ends of a linearly movable valve element in the main control valve and are controlled by the electro-hydraulic valves located elsewhere in the system housing. Such systems also have used bypass/damping valves which operate upon loss of fluid power to bypass flow to and from the main ram or actuator through fixed metering orifices which damp and control the rate of ram and flight surface movements. Like the electro-hydraulic valves, such bypass/damping valves have been located in the system housing remote from the main control valve. In addition, such systems have utilized electronic differential pressure sensors to provide dynamic ram load feed-back information to the aircraft electronic control system which supplies command signals to the electro-hydraulic valves.
An alternative approach to the electro-hydraulic control system is an electro-mechanical control system wherein a force motor is coupled directly and mechanically to the main control servo valve. For an example of one form of such systems, reference may be had to U.S. patent application Ser. No. 326,536, filed Dec. 2, 1981, now U.S. Pat. No. 4,466,597, granted Aug. 21, 1984 and entitled "Electro-Mechanical Direct Drive Valve Servo System with Rotary to Linear Valve Drive Mechanism". These systems also have used remotely located bypass/damping valves which bypass ram flow through fixed metering orifices upon loss of fluid power for damping and controlling the rate of ram and flight surface movements.
In hybrid electro-mechanical systems, the force motor is coupled directly and mechanically to a pilot valve plunger which controls a hydraulically powered servo valve actuator for driving the main control servo valve. As disclosed in U.S. patent application Ser. No. 442,873, filed on Nov. 19, 1982, now U.S. Pat. No. 4,472,988, granted Sept. 25, 1984, and entitled "Redundant Control Actuation System-Concentric Direct Drive Valve" and U.S. patent application Ser. No. 463,631, filed Feb. 3, 1983, now abandoned in favor of a continuation application Ser. No. 680,230, filed on Dec. 10, 1984, and both entitled "Control Actuation System Including Staged Direct Drive Valve With Fault Control", a shut-off valve sleeve concentric with the pilot valve plunger can be used to direct ram flow through fixed metering orifices upon loss of fluid power for damping and rate control of ram and flight surface movements.
As indicated, each of the foregoing systems uses added valves or valve components to achieve some degree of control over ram and flight surface movements after loss of fluid power. This results in increased package size especially in plural redundant systems where redundant valves or valve components are required for multiple hydraulic actuator systems. Furthermore, such valves or valve components are shuttled between system on and off (bypass) positions with the latter serving to direct actuator flow through the fixed metering orifices. Consequently, there has been no provision for active damping or flutter control in response to changing conditions at the ram or flight control surface. Also ram pressure relief flow through the fixed metering orifices may under some circumstances be insufficient to prevent overload of the ram and flight control surface and resultant damage.
With the foregoing in mind, it would be advantageous and desirable to provide for active and more precise load and damping control in an aircraft flight control servo system. Furthermore, it would be desirable to provide for such active or regulated control while minimizing system package size such as by attributing multiple functions to servo system components.